ABSTRACT Mycoplasma genitalium (MG) is a sexually transmitted pathogen, recognized as an important cause of several reproductive tract disease syndromes in men and women. Despite the inflammation and specific antibodies produced by the host, MG infections are often chronic, persisting for many months if not treated with effective antibiotic therapy. We and others have shown that the adhesins of MG, MgpB and MgpC, undergo phase and antigenic variation, likely contributing to evasion of the host immune response, persistence, and reinfection. MgpBC variation is mediated by a unique system of segmental reciprocal recombination between the mgpBC expression site and archived homologous partial copies of these genes, termed MgPars, located throughout the MG chromosome. However, the components required for mgpBC/MgPar recombination and their regulation are incompletely understood, in part due to the difficulty of working with this fastidious bacterium and the absence of a complete set of recombination genes and regulators in MG's tiny genome (580 kb). The goal of this proposal is to characterize the regulatory mechanism(s) mediating mgpBC/MgPar recombination, and therefore antigenic variation, in MG. Recently, we discovered that MG428 positively regulates recombination enzymes (recA, ruvA, and ruvB) and determined that this protein is a previously uncharacterized, extracytoplasmic function (ECF) sigma factor. We have also shown that, unique to MG, RecA is expressed as three distinct isoforms generated by alternative translation initiation: an active, full-length RecA and two inactive, N-terminally truncated versions of this protein. We propose that the small isoforms regulate mgpBC/MgPar recombination in MG by inhibiting the activity of full-length RecA. Further, we have determined that a novel protein of unknown function reduces antigenic and phase variation. We hypothesize that this putative membrane protein controls mgpBC/MgPar recombination in response to environmental signals. In addition, we have determined that growth in low CO2 enhances antigenic and phase variation rates. Building on these preliminary findings, we seek funding to explore the function of the truncated isoforms on full-length RecA activity (Aim 1), define the mechanism by which antigenic and phase variation is reduced by our newly identified membrane protein, including its putative network of interacting partners (Aim 2), and explore how CO2 and other environmental conditions affect this process (Aim 3). Finally we will investigate the effect of these conditions on recombination gene transcription and the expression of RecA isoforms, thereby defining the pathways controlling mgpBC/MgPar recombination in this organism. We anticipate that this study will reveal a novel, complex, and tightly regulated system of antigenic and phase variation mediating survival and evasion of the host immune response in a clinically significant and extremely fastidious pathogen.